A bipolar transistor such as a PNP transistor, for example, may be constructed of a P-type emitter located in an N-epi material. Spaced apart from the emitter by the N-epi material may be a P-type collector, with the N-epi material forming a base for the P type collector and P type emitter. The N epi layer is surrounded by a P+ region for isolation to adjacent N epi regions. An N+ buried layer placed below the P+ emitter and collector regions prevents the flow of carriers from the P+ emitter to the P+ region and P substrate due to the creation of a parasitic transistor formed by the P+ emitter material, the N-epi base material and the P substrate material which acts as the collector.
As would be apparent to those skilled in the art, the saturation of bipolar transistors leads to an excess of minority carriers in the base region. These minority carriers flow to the P+ isolation and P substrate regions which are negatively biased, resulting in substrate currents.
This substrate current may be significant resulting in elevated IC temperature due to increased power dissipation. The current can also result in parasitic latch-up conditions. Both are undesirable effects.